The invention relates to the diffraction of light by a surface grating, and more particularly to a surface grating structure that eliminates or at least reduces the polarization-dependence of the diffraction efficiency of such grating.
Increasingly, information transmitted over long distances increasingly travels over fiber-optic communication systems. It is well known that standard single-mode fiber may not preserve to preserve the launched state of polarization (SOP) of optical signals propagating through the fiber. Moreover, the SOP usually varies with time at any given point along the fiber due to small changes in the physical environment of the fiber or in the optical wavelengths. These random polarization fluctuations can affect transmission systems that employ polarization-sensitive optical components, such as optical amplifiers, coherent optical receivers or polarization-dependent photonic switches and demultiplexers. Polarization scramblers have recently been employed in optically amplified transoceanic communication systems, where they are used, for example, to eliminate anisotropic gain saturation (polarization hole burning) in the optical amplifiers by depolarizing the launched optical signal. Accordingly, optical components used with optical fibers should be made polarization independent, thereby reducing costs and complexity of the fiber-optic communications system.
In wavelength division multiplexed (WDM) systems, different channels are associated with different optical wavelengths. Channel allocations for dense WDM systems are determined by complex engineering tradeoffs relating both to transmission effects and to technology constraints at the transmitter, demultiplexers, and receivers. The international telecommunications union (ITU) standards body has proposed a channel allocation grid with 100 GHz channel spacing (xcx9c0.81 nm at a 1550 nm wavelength) on even 100 GHz intervals, counting nominally in both directions from a center frequency of 193.1 THz. Newer systems are being designed to reduce the channel spacing to 50 GHz or less. In addition, the total wavelength range over which these devices are designed to operate is increasing. Hence, devices are required for separating (demultiplexing) the various wavelengths from a common transmission fiber into separate optical channels and for combining (multiplexing) the various optical channels back into a single fiber. Passive wavelength-selective components include interference filters, fiber Bragg gratings, arrayed waveguide gratings (AWG), and free space gratings, such as surface relief diffraction gratings. Many of these wavelength-selective components have a polarization-sensitive response.
It would therefore be desirable to provide multiplexers and demultiplexers that have a uniform and constant diffraction efficiency over the wavelength range of interest independent of the polarization state of the light.
The invention is directed to an optical surface diffraction grating designed to provide a substantially identical diffraction efficiency over a certain wavelength range for two orthogonal polarization directions of light. According to one aspect of the invention, the grating includes at least one first grating element with a grating period and a first height defining a modulation depth of a first grating relief pattern, and at least one second grating element with the same grating period, and a second height defining a modulation depth of a second grating relief pattern. The second height is different from the first height. The first and second grating elements are arranged substantially side-by-side. The first height and the second height are selected so that an average value formed from the diffraction efficiencies of the first and second grating elements over the preselected wavelength range is substantially identical for two orthogonal polarization directions.
According to another aspect of the invention, an optical wavelength division demultiplexer using the substantially polarization-independent grating device includes at least one optical illumination source and a plurality of optical receiving devices receiving wavelength-selected diffracted optical radiation from the grating device. The characteristic features of the grating device are selected so that the intensity of the wavelength-selected diffracted optical radiation is substantially independent of a direction of polarization of the at least one optical illumination source.
According to yet another aspect of the invention, an optical wavelength division multiplexer using the substantially polarization-independent grating device includes a plurality of optical illumination sources and at least one optical receiving device wavelength-combined diffracted optical radiation from the grating device. The characteristic features of the grating device are selected so that the intensity of the wavelength-combined diffracted optical radiation is substantially independent of a direction of polarization of the plurality of optical illumination sources.
According to another aspect of the invention, a method of producing diffracted optical radiation that is substantially independent of the polarization direction of illuminating optical radiation over a preselected wavelength range is disclosed. The method includes diffracting a first portion of the illuminating optical radiation on at least one first grating element having a grating period and a first height defining a modulation depth of a first grating relief pattern, and diffracting a second portion of the illuminating optical radiation on at least one second grating element having the same grating period and a second height different from the first height and defining a modulation depth of a second grating relief pattern. The diffracted first and second portions are combined to form the diffracted optical radiation. The first height and the second height, and the first portion and the second portion, respectively, are selected so that an average value formed from the diffraction efficiencies of the first and second grating elements over the preselected wavelength range and the illuminated area is substantially identical for two orthogonal polarization directions. Such polarization-insensitive multiplexers and demultiplexers can be applied in wavelength division multiplexed (WDM) fiberoptic telecommunication systems.
Embodiments of the invention may include one or more of the following features. The average value of the diffraction efficiency may be weighted according to a ratio of surface areas of the first and second grating elements exposed to optical radiation illuminating the grating device. To obtain a substantially wavelength-independent diffraction efficiency, the first height and the second height can be selected so that a slope of the diffraction efficiency of the first grating element as a function of wavelength over the predetermined wavelength range is opposite in sign from a slope of the diffraction efficiency of the second grating element over the predetermined wavelength range. The grating elements may be ruled gratings, in which case the respective heights of the first and second grating relief patterns define respective blaze angles, or the gratings may be fabricated by holographic means. The first and second grating elements may be arranged in a geometrical pattern on a common surface, which can be flat or curved, for example, concave to provide focusing. The geometrical pattern may be a stripe or checkerboard pattern or any other suitable pattern. The grooves of the grating elements may be aligned parallel or at an angle with respect to areas where the different grating elements are joined. The area where the grating elements are joined may be apodized.
Additional features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.